Flicker-photometer.



No. 815,800. PATENTED MAR. 20,1900.

J. T. DBMPSTER. PLIGKER' PHOTOMETBR.

APPLICATION FILED AUG. 22,1904.

2 SHEETS-SHEET 1.

Witnesses Invento r j John T. Dempster,

\gZ I by v photometer-bar whichiwill JOHN THos. DEMPSTER, OF GENERALELECTRIC SCHENEOTADY, COMPANY, A CORPORATION F NEW YORK.

UNITE STATES PATENT o EIoE.

NEW YORK, AssreNoR TO 'FLSICKERS-PHOTOMIETERP Specification ofLettersPatent.

Application filed August 22, 1904. Serial No- 22l,668.

Patented March 20, 1906.

' Photometric comparisons of light sources are often renderedinconvenient and inaccurate because of the difierence incolor betweenthe lights compared. Thus in comparing an arc-lamp with an incandescentstandard by means of a Bunsen or grease-spot photometer thedifference-in color between the two lights makes it practicallyimpossible to determine a setting of the photometer on the make thecontrastedsurfaces of the'photometer-screen appear eq'uallyilluminated.In such cases the operator must depend to a largeexte'nt upon h1sexperience and judgment inthe comparison of light intensities, and sincethe contrast in color affects different observers in different degreesthe photometricresults obtained by such a'comparison .are oftenunreliable.

The object. of the presentQinvention is to "provide a means forreadilyand accurately" comparing. light sources irrespectivefof the color ofthe light. .Thephotometer described herein embodies the principle, wellknown-in optics, that if the human eye is alternately subjected to lightfrom two sources, the exposures followi each other in rapid succession,the eye will experience a peculiar flickering sensation; "due to thereflex'action of the nerves of the eye. in attempting to adjust thefocus of the eye to correspond with. the intensity of the ight received;but if the two light sources are ra'dually brought to the sameintensitythe %ickerin sensation gradually disappears. This fiic er is dependententirely upon the intensity of the light'and ap ears to be uninfluencedby the color of the ight transmitted tothe eye, so::that a photometerembodying the prlnciple may be used for'a comparison of lights radicallydifferent in color, as well as for lights of thesame qualityw Theinvention Will be readily understood by reference to the followingdescription, taken in connection with the drawings forming a part ofthis specification, in which-v Figure 1,is a perspective viewof thephoprefer to make use of This sensation, is probablyv cave slottedwasher 24 is tometer with the movable 1 parts rotated vthrougha slightangle about a vertical axis for convenience of illustration. Fig. 2 is aside elevation, partly in section and Fig.3 is a detail of the novelform of friction-bearing used in supporting the sight-tube yoke to thevertical standards. o c

The base block 1 is adapted to support the photometer on the guide-bars2 and 3 and may be adjusted longitudinally thereon by the rope 4,clamped to the base by set-screw 5. Rigidly screwed to the block 1 isabaseplate 6, on'which rests the top plate 7. An annular about avertical axis. Screwed to the topof plate 7 is the plate. 9, whichserves to support the two standards 10 and 11, these stan 1ts drivingmotor. comprises a central'web 12, on the perinhery of which are securedtwo frustoconica surplate 8, rigidly connected to the top I plate7,engages a groove in the lower surface of plate6,.so that the top plate 7and the parts carried thereby may be freely rotated ards serving tosupport the. photometric disk and The photometric disk faces13 and-14.Each of these surfaces occupies one-halfthe peripheryof the web 12,andthe elements of the surface make an an- .gle of forty-five degreeswith the central horizontal axis of the disk. These surfaces serve asreflectors in ;a manner hereinafter described and have a uniformmat-surface. This maybe obtained in many Ways; but I whiteblotting-paper cut in strips15 16 to conform to the surfaces 13 and 14and glued thereto. The web 12 is rigidly clamped to the hub 41 by thenut 42, the hole being clamped to the shaft 17 by set-screw 18. Theshaft 17 is supported in standards 10 and 11 by a bearing of novelconstruction comprising a cylindrical hub 19,

having an annular. flange 20 onits inner edge and a groove 21 near itsouter edge, the bot tomof said groove having a sloped surface;

Anannular ring 22 is adjustably secured near the outer end of thecollar-by set-screws 23 theinner ends of the set-screws engaging withthe sloped surface of the groove, so that by tightening the set-screwsthe ring may be forced longitudinally on the collar. I

pressed by ring 22 against the standard 10. By the means described thearms 25 and 26 of the sight-tube vyoke are clamped securely to thestandards 10 and 1,1, but at the same time the securing A conmeans issufficiently resilient to permit rotation of the sight-tube 27 about ahorizontal axis coincident with the shaft 17. Circular openings 28 areprovided in the yoke-arms 25 and 26 to permit the passage of the lightfrom the sources to be compared to the photometric disk, from which itis reflected through the sight-tube 27 to the eye of the operator at theeyepiece 48. As the reflecting-surfaces are symmetrically arranged onthe rotating member, the length of the refiected ray is constant. Thelight-tube 27 is screw-threaded in the crossbar 29 of the yoke forconvenience in packing for shipment. The driving-motor 30 may be of anyconvenient type and is direct-connected to the shaft 17 of thephotometer-disk. To secure freedom of rotation of the standards 10 and11 about a vertical axis through the center of base-plate 6, a specialarrangement of the motor connections is necessary. The terminal 31 isconnected by an insulated conductor to the brass stud 32, which passesdown through an annular insulating-plug 33 to a small cavity in theblock 1, in which islocated contact-brush 84, said brush being elec--trically connected with binding-post 35. The other terminal of the motoris grounded directly to the motor frame, the frame being connected withbinding post 36. In order that-sight-tube 27 may be at exactly rightangles to the photometer-bar when readings are being taken, the topplate 7 is'provided with a small conical depression 37 on its undersurface, into which fits a plunger 38, adjustably supported by coilspring 39, contained in a recess in the base-block 1. The top plate 7 isprovided with a second conical depression 40, located diametricallyopposite the depression 37, so that when the sighttube 27 is rotatedthrough one hundred and eighty degrees it will still have the properangular relation with the photometer-bar.

The operation of the device is as follows: The two lights to be comparedare placed at the opposite ends of the photometer-barin the ordinarymanner, and the rotatable disk ofthe photometer is set in motion by itsdriving-motor. The speed of the disk appears to have little influence onthe accuracy of the measurements within wide limits; but two hundredrevolutions per minute gives good results. The light from the twosources passes through the openings inthe yokearms 25 and 26 andencounters the rotating disk, from which the light from onesource isreflected through the sight-tube 27 to the eye of the observer, whilethe light from the other source is reflected back at an angle to thephotometer-bar and does not enter the sighttube at all. At eachhalf-revolution of the disk a new reflector-surface comes before the endof the sight-tube, thus alternately presenting to the eye of theobserver the reflected light from the two sources. As the change fromone source to the other is exceedingly abrupt because of the shape ofthe conical reflector-surfaces and because of the way in which they aremounted on the web 12, the effect on the eye will likewise be abrupt,and any inequality in intensity may be readily detected by theflickering sensation produced. To determine the candle power of thelight, the photometer is adjusted along the bar until the flickerentirely disappears. The location of the photometer then indicates thepoint of equal intensity of illumination, and the calculations of candlepower may be made by the ordinary method. To compensate for anyirregularity in the refleeting-surfaces 15 16, the top plate 7, togetherwith the parts supported thereby, 1s rotated through an angle of onehundred and eighty degrees, thereby presenting the reverse side of therevolving disk to the standard light and changing the sight-tube 27 tothe opposite side of the photometer-bar.

In order that all readings may be taken from the same side of the bar,the sight-tube is then rotated through one hundred and eighty degreesabout the shaft 17, thus bringing it to its original position to thefront of the photometer-bar, in which position a second reading istaken. The friction between the yoke-arms 25 and 26 and the standards 10and 11 while sufficient to support the sight tube 27 in a horizontalposition will nevertheless permit this movement of the yoke.

\Vith the exception of the reflecting-surfaces 15 16 all the parts arefinished in dull black, so that stray light will not in any wayinfluence the accuracy of the readings.

What I claim as new, and desire to secure by Letters Patent of theUnited States, is-

1. A flicker-photometer having a plurality of movable light reflectingsurfaces, and means for successively viewing said reflecting-surfaces.

2. A flicker-photometer having a rotatable member, a plurality oflight-reflecting surfaces carried thereby, and means for successivelyviewing said reflecting-surfaces.

3. A flicker-photometer having an eyepiece, a plurality oflight-reflecting surfaces for reflecting lightfrom different sources tosaid eyepiece, and means for successively moving said surfaces topositions equidistant from said eyepiece.

4. In a flicker-photometer, a rotatable disk having frusto-conicalreflecting-surfaces the elements of which make an angle of fortyigvedegrees with the axis of rotation of the is i.

5. A flicker-photometer having a rotatable disk, means for rotating saiddisk, reflectorsurfaces on said disk to reflect light from two differentsources, means for reversing said disk to interchange thereflecting-surfaces with respect to the lights, and means for viewingsaid reflecting-surfaces.

disk,;refiector-surfaces on said diskto reflect light from two differentsources,lstandards' for su porting said disk, a sight-tubesupporte from[said standards,

spect to the'light soureesa p ,7. A flicker-photometer havingarotata'ble disk, reflector-surfaces on said disk to reflect light fromtwo different sourcesystandards for suipgortingsaiddisk, a sight-tubesupp'orte v om said standards, means whereby hundred and eighty degreesto. interchange the reflector surfaces with respect to'v the I lightsources, and means for rotating the o eyepiece for viewing said disk.

sight-tube througha vertical. angle of one hundred and eighty degrees.-

i 8. In a flicker-photometer, a rotatable disk having tworeflecting-surfaces on its periphery, each of said surfaces coveringsubstantially half the periphery of the disk and being. of substantiallyuniform width. I

9. A' flicker-photometer having a baseblock, standards, pivotallymounted on said block, a rotatable disk carried by said standards, meansfor rotating said disk, and an and .means 1 whereby 'said standards maybe rotated through one hundred and eighty degrees .to interchange thereflectorsi lrfaces with re- 10. 'fiickerphotometer having a baseablock, standards mounted onsaidblock, a

rotatable disk-supported by said standards,-

said disk having. angularly-disposed reflect ing-surfaceson its.peripher means for rosaid disk.v I y p 11. A flicker-photometer having abase- *blook, standards mounted on said block, a

rotatable disksupportedby said standards,

said diskhaving a central weband frustoconical reflecting-surfaces onthe periphery of the web, means forrot'ating said disk, and

\ v p I an eyepiece for'viewing said disk. said standards may be rotatedthroughone 12-. A flicker-photometer having a plurality -ing saidreflecting-surfaces, and means for of light-reflecting surfaces, meansfor rotat V successively viewing said surfaces While in motion.

13. A flicker-photometer having a plurality I of light-reflectingsurfaces, means for 0on tinuously moving said surfaces at equal speeds,and means for successively viewing said surfaces While in motion.

' Inwitness whereof I have hereunto set my hand this 19th day of August,1904. L

- V 1 JOHN THOS. DEMPSTER.

' Witnesses: I BENJAMIN B. HULL,

' HELEN OEFORD.

:5" .tating said disk,- and an eyepiece for viewing w

